The present invention relates to fluid coupling devices, and more particularly, to such devices which are capable of changing between the engaged and disengaged conditions, in response to variations in a predetermined condition such as engine compartment temperature.
A fluid coupling device of the type to which the present invention relates typically includes an input coupling member and an output coupling member. The output coupling member cooperates with a cover to define a fluid chamber, and a valve plate separates the chamber into a reservoir and an operating chamber. The input coupling member is rotatably disposed in the operating chamber, with its front face adjacent the valve plate. The rear face of the input coupling member, and the adjacent surface of the output member form a series of interdigitated lands and grooves, defining a shear space therebetween.
The valve plate includes a valving arrangement operable in response to variations in a condition such as temperature to permit fluid to flow from the reservoir, through the valve plate inlet port into the operating chamber. Typically, such fluid couplings include a discharge port defined by the valve plate and disposed near the outer periphery of the operating chamber, with some form of pumping element disposed adjacent thereto, and within the operating chamber, such that a small quantity of fluid is continually pumped from the operating chamber back into the reservoir even while the coupling is engaged. The overall torque transmitting capability of the coupling is related to the quantity of fluid in the shear space, and therefore, it is important to achieve a proper balance between the rate of fluid discharge from the operating chamber, and the rate of inlet flow into the shear space, when the coupling is in the engaged condition.
In conventional fluid coupling devices, it is believed that fluid flows one of two probable flow paths from the inlet port to the shear space, or a combination thereof. One possibility is for the fluid to flow radially outward along the front face of the input member, then along the OD of the input, and finally, radially inward to the shear space. Another possibility is that the fluid flows radially outward along the front face of the input member until an annulus of fluid builds up which extends far enough radially inwardly to communicate with one or more circulation holes defined by the input member, such that fluid flows axially through such holes, then radially outwardly into the shear space through a plurality of radial channels passing through the lands and grooves of the input and output members. In view of the affect of the abovementioned radial channels on fill time, the latter mentioned flow path would seem to be more likely.
Among the problems associated with devices of the type described above is an insufficient peak speed of the output member. This condition can occur if the rate of inlet flow, relative to the rate of discharge flow is insufficient to maintain sufficient fluid in the shear space or if, as has been hypothesized, too much of the fluid in the operating chamber remains between the front face of the input and the valve plate. Also, if a fluid of higher viscosity is used in an attempt to increase the torque capacity of the coupling, sufficient filling of the shear space becomes even more difficult.
Another problem area, also related to the filling of the shear space, is the characteristic known as "response time". As used herein, the term "response time" refers to the ability of the coupling device to go from the disengaged condition to the fully engaged condition in a rapid and predictable manner in response to the opening of the valving. By way of example, when the coupling device is being used to drive the radiator cooling fan of a vehicle engine, it is important for the fluid to fill the shear space quickly and sufficiently in response to the valving starting to open when the temperature approaches the maximum allowable, in order to bring the coupling to peak speed and prevent overheating of the engine. It is also desirable, when the valve closes, for the fluid to be pumped out of the shear space and back into the reservoir quickly, returning the coupling to the disengaged condition, to prevent unnecessary and wasteful power consumption by the coupling.